The switching fabric of a switching node can be a common memory switching fabric, a conventional space switch, or a rotating-access switch. A common memory switch has limited scalability and may be used only if it is determined that the capacity limit of the switch is appropriate for a given network. A conventional space switch with input buffers provides better scalability and has been extensively studied in the prior art. The scalability of a conventional space switch is determined by two factors. The first factor, and the more severe of the two, is a difficulty of scheduling, which is traditionally based on arbitration among input ports vying for the same output port. The second factor is the quadratic fabric complexity of the space switch where structural complexity increases with the square of the number of ports.
Rotating-access switches are described in U.S. Pat. No. 5,168,492, issued on Dec. 1, 1992 to Beshai et al. and U.S. Pat. No. 5,745,486, issued on Apr. 28, 1998, to Beshai et al. An extension to handle variable-size packets is described in Applicant's U.S. patent application Ser. No. 09/244,824, filed on Feb. 4, 1999, and titled “Rate-Controlled Multi-Class High-Capacity Packet Switch”.
A rotating-access switch comprises an array of input ports each having an input memory, an array of output ports, a bank of transit memory devices, an input rotator and an output rotator. The input rotator cyclically connects each ingress port to each transit memory and the output rotator cyclically connects each transit memory to each output port. Each transit memory is logically divided into a number of segments; one segment associated with each output port. The switch operates by cyclically storing data for a deterministic period of time in the transit memory devices. The input ports write data from their input buffers to segments in the transit memory. During each cycle of the output rotator, the output ports read data segments from corresponding memory segments of the shared transit memory devices.
The rotating-access switch is believed to provide significant structural, control and scalability advantages over a conventional space switch. In particular, a rotating-access switch greatly simplifies multi-cast communication in a large-scale node and may scale from a capacity of a fraction of a terabit per second to several hundred terabits per second.
The capability and efficiency of a switching network are determined primarily by its switches and, because of this pivotal role of the switches, switch design continues to attract significant attention.